Alexandr Dolgopolov, the right-handed Ukrainian with no major titles and fluctuating intensity, and Rafael Nadal, the left-handed Spanish great whose inner flame seems to be set permanently to full burn, are very different tennis players and competitors.

But they shared the same reaction this autumn in Asia when confronted with the cruel beauty of a Juan Martín del Potro forehand winner from an unexpected angle.

Dolgopolov was playing the big Argentine in the Tokyo quarterfinals; Nadal in the semifinals in Shanghai. As del Potro unloaded on the stretch as only del Potro can, both men jerked their heads back on the run in disbelief at the result.

It is one thing to elicit a collective gasp from a crowd full of enthusiastic amateurs; quite another to earn shock and awe from fellow professionals for whom flashy winners are the coin of the realm.

But the del Potro forehand has the force and consistent depth to rock just about anyone’s world, and it is the shot that could disrupt the leadership of men’s tennis, both at the ATP World Tour Finals next week in London, where del Potro once reached the final, and beyond.

“To me, next year is the year,” said Brad Gilbert, the former top-10 player who has since coached Andre Agassi, Andy Roddick and Andy Murray. “If we’re going to see Delpo make his move, it’s next year, because to me there’s no reason he shouldn’t ascend into the top 4, top 3, winning a major.”

Del Potro won a major at the 2009 United States Open, routing Nadal in the semifinals and then coming back to beat Roger Federer to take the title. By January 2010, he was No.4 in the world at age 20 but wrist surgery and the rise of Murray and Novak Djokovic to join Nadal and Federer in the oligarchy created a barrier to trading up to his former place.

He is back on the brink now, ranked No.5 and having won two of his last three tournaments with the loss coming in a terrific three-setter in Shanghai to Djokovic that was a condensed version of their classic five-setter in this year’s Wimbledon semifinals, in which Djokovic also prevailed after great effort.

But del Potro is still the only man to have beaten Djokovic, Nadal, Murray and Federer this season and his 6-2, 6-4 dismantling of Nadal in the Shanghai semifinals was a particularly convincing display with the forehand inflicting much of the damage.

Nadal’s whipping forehand remains the more versatile, consistent weapon and at this stage is still the most feared groundstroke in the game. But del Potro’s is the bigger forehand — able to explode crosscourt or down the line as he extends — and it is not as if he is swinging with all the might in his 6-foot-6, or 1.98-meter frame.

“He’s got really nice timing, nice long swings, and really generates a lot of speed without taking what I would call a violent swing at it,” Gilbert said. “Fernando González would hit it unbelievably big, but he would look like he would come out of his shoes.”

At his height, del Potro has long levers with which to produce power, but there are a lot of tall men in elite tennis today and nothing quite like the del Potro forehand.

“He makes contact with the ball relatively far away from his body, which means he has a bigger swing arc,” said Rob Koenig, the former South African player who is now one of the busiest commentators in the sport. “Coupled with that, his arm is almost dead straight when he makes contact with the ball. Only he and Roger are similar in that department because he has a semi-western grip. The more western the grip, the harder it is to keep the elbow and arm fully extended on contact. You’ll hit more spin but won’t generate more power.”

In March, John Yandell, a Yale-educated tennis coach and analyst, calculated that Nadal’s forehand averaged 3,300 revolutions per minute while del Potro’s averaged just 2,000.

“His ball is flatter, and your first thought seeing an Argentine is you think of them having the heavy spin,” Gilbert said. “But his game is not typical South American.”

That is partly because del Potro grew up playing on hard courts in the provincial city of Tandil instead of the clay courts of Buenos Aires.

But he will be in London for the next few days, and based on current form and forehand, no one will want to play him. Perhaps not next year either.